1. Field of the Invention
The present invention relates to fluid handling apparatus, and more particularly to marine tanker loading arms with systems for counterbalancing the arm members.
2. Description of the Prior Art
Various types of fluid transferring devices are used in conveying fluids between a dock, buoy, or floating barge and a tanker or other marine transport vessel. Some of these devices are large articulated pipe structures referred to as marine loading arms, such an arm generally comprising an upstanding support riser on which is pivotally mounted an assembly of two or more arm members that are pivotally interconnected by swivel joints in a manner to allow the arm to accommodate itself to the movements of the buoy, barge and/or tanker while fluid transfer is in progress. Some examples of these prior art fluid transferring devices are described in U.S. Pat. Nos. 2,980,150 and 3,382,892 to Bily, 3,581,769 to Haley, and 3,889,728 to Riche.
It is common practice to install weights to counterbalance a loading arm about its horizontal pivot axis at the riser so that it can be more easily maneuvered. To achieve a balanced condition, some marine loading arms are provided with a support beam that is secured to the inner arm member and extends rearwardly therefrom behind the riser, and with a counterweight fixedly mounted on an inner sheave that is rotatably mounted on the support beam and connected by an endless cable to an outer sheave located at the juncture of the inner and outer arm members. The outer sheave is fixed to the outer arm member but rotatable with respect to the inner arm member, so that the counterweight, sheaves and cable balance the inner and outer arm members in all operating positions.
Although such a single-counterweighted balance system functions generally very well, one of its disadvantages is that the counterweight cannot compensate for changes in the weight of the arm members that occur as these members are filled with, or emptied of, fluid during start up and termination of the fluid transfer operation.
Another disadvantage with some of the prior art counterbalancing systems is that the outer end of the arm cannot be moved rapidly away from a marine tanker in an emergency disconnecting operation, especially if power to the arm has been disconnected or lost.